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A numerical model has been developed for atomic hydrogen densities in the Titan hydrogen torus. The effects of occasional collisions were included in order to accurately simulate physical conditions inferred from the Voyager 1 and 2 Ultraviolet Spectrometer (UVS) results of Broadfoot et al. (1981) and Sandel et al. (1982). The model employed Lagrangian perturbation of orbital elements of hydrogen atoms launched from Titan and Monte-Carlo simulation of collisions and loss mechanisms. The torus is found to be azimuthally symmetric with the density sharply peaked at Titan's orbit, and decreasing rapidly in the outward and perpendicular directions and more gradually inward from 17 to 5 R(s). The energetic hydrogen atoms from Saturn's upper atmosphere, first predicted by Shemansky and Smith (1982), were also investigated. Collisions of these Saturnian atoms with the torus population do not contribute to the torus density, and will lead to a net loss of torus atoms if their launch speeds from Saturn extend above 40 km/sec. The Saturnian atoms produce a corona which was modelled using the theory of Chamberlain (1963). Based on the energetic hydrogen production rate given by Shemansky and Smith (1986), the coronal density at Saturn's exobase is taken to be 200 to 300 cm⁻³, decreasing to 3 or 4 cm⁻³ at 20 R(s). Without the coronal population, the torus model does not reproduce the Voyager 2 UVS Lyman α intensities because the hydrogen atoms are too closely confined toward Titan's orbital plane. The observations can be reproduced by a model that includes the corona and has central plane maxima of 62 cm⁻³ at Titan's orbit and 318 cm⁻³ at Saturn's exobase. The effect of Titan's exospheric temperature (T(E)) on torus structure is seen in the column abundances perpendicular to the central plane at radii of 5 to 15 R(s). Spacecraft observations of these column abundances should allow verification of T(E) to within about 100°K. Similar observations of other species expected to be present in the torus, such as H₂, N, and N₂, would indicate their approximate launch speeds from Titan and thus the relative importance of thermal and non-thermal loss mechanisms.

In 1979, the IRIS infrared spectrometers on the two Voyager spacecraft obtained over 1000 disk-resolved thermal emission spectra of Europa, Ganymede, and Callisto, Jupiter's three large icy satellites. This dissertation describes the first detailed analysis of this data set. Ganymede and Callisto subsolar temperatures are 10°K and 5°K respectively below equilibrium values. Equatorial nighttime temperatures are between 100°K and 75°K, Callisto and Europa being colder than Ganymede. The diurnal temperature profiles can be matched by 2-layer surfaces that are also consistent with the eclipse cooling observed from earth, though previous eclipse models underestimated thermal inertias by about 50%. Substrate thermal inertias in the 2-layer models are a factor of several lower than for solid ice. These are 'cold spots' on Ganymede and Callisto that are not high-albedo regions, which may indicate large thermal inertia anomalies. All spectra show a slope of increasing brightness temperature with decreasing wavelength, indicating local temperature contrasts of 10-50°K. Callisto spectra steepen dramatically towards the terminator, a trend largely matched with a laterally-homogeneous model surface having lunar-like roughness, though some lateral variation in albedo and/or thermal inertia may also be required. Subsolar Ganymede spectra are steeper than those on Callisto, but there is no steepening towards the terminator, indicating a much smoother surface than Callisto's. The spectrum slopes on Ganymede may indicate large lateral variations in albedo and thermal inertia. A surface with similar areal coverage of dark, very low thermal inertia material, and bright material with thermal inertia a factor of 2-3 below solid ice, fits the diurnal and eclipse curves, and (less accurately) the IRIS spectrum slopes. Europa spectra have very small slopes, indicating a smooth and homogeneous surface. Modelling of surface water ice migration gives a possible explanation for the inferred lateral inhomogeneities on Ganymede. Dirty ice surfaces at Jupiter are subject to segregation into high-albedo ice-rich cold spots and ice-free regions covered in lag deposits, on decade timescales. Ion sputtering and micrometeorite bombardment are generally insufficient to prevent the segregation. The reflectance spectra of Ganymede and Callisto may be consistent with this type of segregated surface.

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